Deep sea level drillings, for example, down to 1,000 and, in particular, down to 2,000 m, requires that the architecture of the current risers be reviewed.
The difficulties caused by the transport, maintenance, storage and implementation of numerous elements comprising a riser, the resistance of the tubes of connectors and floats subjected to extremely high static and dynamic stresses which are sometimes hard to detect, as well as the manufacture, use and maintenance of a large volume of mud, would to a large extent reduce the effectiveness, reliability and safety of the drilling system and operations.
The words riser, extension tube or standpipe are understood here to mean a pipe making it possible to transfer, in particular, fluids between the water bottom and an installation situated at an upper level, namely one which may be situated roughly at the water surface or be immersed.
The present invention provides a riser making it possible to work at a great water depth by overcoming the difficulties previously indicated without adversely affecting the effectiveness or reliability of the riser.
For several years, operators have been constantly increasing the diameter of the riser (16"-185/8- 21) and the BOP working pressure (10,000-15,000 series), the same applying to the use of high density mud (maximum density in question in this analysis being 17 ppg, namely 2.03). This evolution, justified by reasons of safety and effectiveness, more particularly adversely affects the behaviour of the extension tube.
The problems linked to the static dimensioning of the riser appear when a riser longer than 1,000 m is dimensioned according to the prior art.
It has been observed that the internal volume of the riser is greater than the volume of the well itself as soon as the water depth goes down to 1,000 m (for example, the maximum volume of the well, which down to a water depth of 1,246 m, is equal to 164 m3, whereas, the volume of the riser reaches almost 200 m3). It exceeds by double or even triple this volume at a water depth of 3,000 m. This however, presents many serious technical and economic difficulties for the manufacture, storage, maintenance, control and treatment of mud and, finally, there remains the question of safety concerning such drillings.
As the apparent weight of this mud needs to be entirely supported by the tensioners, it follows that the greater the water depth, the greater is the pressure at the top of the riser, even if the actual weight of the riser is nonexistent (in the water).
Consequently, this firstly requires that an installation aboard the surface units has a sufficient tensioning capacity, with regard to the recommendations stipulated in the standards (standard API RP 2Q) currently in force, and secondly the use of tubes which are thicker at the upper portion of the riser so that the stresses there are limited to acceptable values.
Such a dimensioning shows that the ratio between the minimum tension required at the top and the available tensioning capacity would, in many cases, exceed the 71% authorized by the API RP 2Q standard, whereas, the apparent weight of the riser would be assumed as being equal to 0, which conforms neither to reality nor desirability, as it shall be seen further.
Finally, the differential tension generated in the lower section of the riser by virtue of the difference between the maximum density of the mud (dmud=2.03) and that of the seawater (dsw=1.03) here also requires the use of highly-resistant tubes.
These observations actually show the difficulties brought about by the simple static dimensioning of the riser due to the increase in weight and pressure of the mud which is contained there when the water depth increases. It might be imagined that these difficulties could be overcome by simply using sufficiently thick tubes and relatively resistant connectors. Unfortunately, this is not, strictly speaking, true due to the consequences on the floats and, secondly, a worsening of the dynamic behaviour of the riser.
As regards the problems linked to the floats, the following observations may be made:
The overhead minimum tensions mentioned above have been determined, as already stated, by supposing that the weight in water of the riser elements was nil. Such a hypothesis implies that a sufficient number of floats is tied to so as to fully compensate for the weight of the tubes and of the connectors which form them.
Now, it is known that the density of synthetic foam floats, which ordinarily equip the riser elements, is that much greater when the water depth level (and, consequently, the water pressure they need to resist) is high. Below 2,000 m (200 bars), the resistance of these floats is moreover not yet proven and the corresponding costs concerning development, all the more greater since the water depth level is high. qualification and provisioning could be quite high.
Secondly, the external diameter of the floats needs to be less than a certain value, namely about 1.2 m (47 inches), so as to enable the floats to pass into the 125.7 cm (491/2 inches) turntable during the maneuvering of the riser. The volume of the floats and, accordingly, the induced buoyancy are thus physically limited. Therefore, in practice, there is a depth below which the synthetic foams no longer fully compensates for the weight of the riser, with this lack of buoyancy needing to be made up for by a greater tensioning installed capacity. This depth may be estimated at about 2,000 m.
As regards the problems linked to the dynamic behavior of the riser, many factors combine in deteriorating the dynamic behaviour of risers when the water depth level increases.
More particularly, when the riser, connected to the well head, is supported by the steady tension cables of a heave compensation system, it has been observed that the dynamic response of this system is less effective when the tension applied is greater. In other words, for a given mud density, the rigidity of the system increases along with the water depth level. It results, at constant heave in an increase of the amplitude of the oscillations of the tension (around the mean tension) in all the sections of the riser which needs to be taken into account in the fatigue resistance calculations of the tubes and connectors.
However, it is when the riser, which is not connected to the top of the well, is suspended under the floating support without decoupling the relative heave movements of this riser and the support in the event of a storm or during operation that the situation is the most troublesome.
Increasing the riser length actually results in an increase proportional to at least its weight. If, in addition, it is firstly considered that the tubes and connectors need, for those reasons specified above, to be more resistant and thus heavier and that, secondly, the higher the density of the floats attached to the tubes and connectors, the greater the ambient water pressure in the floats need to resist, one can clearly understand that the proportionality factor is in fact greater than 1.
This rapid increasing of the riser weight when the water depth increases provokes the emergence of two phenomena, not very significant and often ignored for average and low water depths. These phenemona may then condition dimensioning, and their characteristics, causes and effects need to be carefully analyzed.
More particularly, the increasing of overtensionings due to inertia of the riser during violent storms may provoke the partial or total detensioning of the upper part of the riser and induce there redhibitory bending stresses in correlation with the other movements (breakout movement, skidding) and the direct action of swell.
Additionally, the raising of the actual period of longitudinal vibrations beyond values for which the amplitude of heave is nil could considerably limit, even in relatively calm weather, the maneuvering operations of the riser to the risks it would present.
The problems linked to control of eruptions and to drilling safety are definitively of less importance concerning the conception and dimensioning of the riser with respect to the existing situation for smaller water depths. As regards this field and when the water depth increases, the following stress principles apply.
The working service of the "kill and choke lines" raised to 1,050 bars (15,000 psi) requires the use of thicker tubes which contribute in increasing the weight of the riser and accordingly increasing the problems linked to its dynamic behavior (see above).
The increasing of head losses induced in these lines during eruption control operations renders these operations more delicate and dangerous to carry out.
The distance of the well head and the intense pressure existing in the head render it more difficult to detect the possible presence of gas in the well and act accordingly by closing the valves of the valves wedges in time. The presence of large quantities of gas in the central tube of the riser thus becomes more likely.
This is expressed by the increased need of using materials fully resistant to the corrosion provoked by petroleum effluent and especially by sulphured hydrogen (H.sub.2 S).
This also results in the risk of seeing this gas, once decompressed, partially or completely filling the inside of the main tube whose wall, not being thick enough to resist the ensuing hydrostatic compression, would not be able to avoid collapse.
The final type of the problems listed above concerns the difficulty of storing on the bridge of the floating supports the large number of elements comprising the riser when its length increases and the factors need to be considered.
Namely, the resistance of the structure of the platform under the storage areas needs to be sufficient so as to support the total weight of these elements (see above).
Moreover, the volume occupied by the elements, having regard to the size of the floats, needs to be compatible with the available locations and with the naval stability of the rig.
Furthermore, the disposition and handling of the riser shall enable it to implement, according to a specifically established order, elements with different characteristics (central tube thickness, density of floats, with etc), these elements being more numerous (4 or
When the water depth is greater is greater. The use of means for the automatic handling of the riser elements, becoming increasingly common on modern rigs, needs to be considered.